systemd: add option to use stub-resolv.conf

systemd: add option to use stub-resolv.conf

Add option to use the stub-resolv.conf file, which is the systemd
upstream's recommended default mode
https://www.freedesktop.org/software/systemd/man/systemd-resolved.service.html#/etc/resolv.conf

This enables the resolution of Multicast DNS and Link-Local Multicast
Name Resolution names for programs that do not use Name Service Switch.

Signed-off-by: Eero Aaltonen <eero.aaltonen@vaisala.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>